Order and disorder in ultrathin Pb films grown on Si(1 1 1) 7×7 substrates at low temperatures

The growth of Pb on Si is of special interest due to the formation of metallic monolayers on a substrate, which is insulating at low temperatures. Since the knowledge of structural details is decisive for the interpretation of electronic properties, many studies have been made, which unfortunately did not provide a consistent description. Especially studies at liquid He temperatures are needed for the interpretation of electronic studies in this temperature range. For that purpose Pb has been deposited onto Si(1 1 1)7×7 at 25 K. During deposition, annealing and after cooling the structure and especially the defect structure has been carefully studied using high resolution low-energy electron diffraction for spot profile analysis (SPA-LEED). Several unexpected features show the special situation of ultrathin Pb films. The first three monolayers are heavily disordered, so that no LEED pattern is observed in the full energy range of LEED. Nevertheless oscillations of the LEED intensities are observed for coverages θ>4 ML. Therefore, a recrystallization to an epitaxial structure occurs even at 25 K. Rotational disorder is derived from an azimuthal broadening of the Pb diffraction spots. Improved epitaxial quality films with larger domains and no azimuthal broadening are observed, when at first 2D islands are formed due to annealing at room temperature and further deposition is performed at low temperatures.